1. Technical Field of the Invention
The present invention relates generally to a radar apparatus designed to transmit a continuous wave for detecting a target object, and more particularly, to a radar apparatus designed to process beat signals to form digital beams using the complex Fast Fourier Transform (FFT) for frequency analysis of the beat signals.
2. Background Art
Automotive radar systems are known which are designed to measure the distance to, the azimuth, and the relative speed of an object present ahead of an automotive vehicle for cruise control and/or anti-collision control. Radar systems of this type usually use as a radar wave a continuous wave (CW) or a frequency-modulated continuous wave (FM-CW). The radar systems receive a return of the radar wave from an object, mix it with a local signal having the same frequency as that of the transmit signal (i.e., the transmitted radar wave) to produce a beat signal whose frequency is equivalent to a difference in frequency between the received signal and the local signal, and analyze the frequency of the beat signal to derive information on the target object.
Specifically, the return of the radar wave undergoes the Doppler shift as a function of the speed of the radar system relative to the target object. The travel of the radar wave to and from the target object causes the received signal to be changed in frequency and phase from the transmit signal. This change appears at the beat signal. The distance to and relative speed of the target object may, thus, be determined by analyzing the beat signal.
The detectable range of the radar system is defined geometrically by a beam emitted from or received by an antenna. It is, thus, advisable that multiple beams be provided in order to increase the detectable range without decreasing the detectable distance to a target or determine the azimuth of the target in the limited detectable range. The provision of the multiple beams is typically achieved using a plurality of antennas oriented in different directions or a known phased array, but in recent years when it has become possible to process digital signals at high speeds, attention is being paid to digital beam forming (DBF) which forms a plurality of beams through digital signal processing.
FIG. 8 shows one example of conventional radar systems adapted to produce beams using the DBF technique.
A transmitting antenna AS installed in a transmitter 104 radiates a radar wave. A return of the radar wave from an object is received by a plurality of receiving antennas AR1 to ARN simultaneously. A receiver 106 mixes a signal outputted from each receiving antenna ARi (i=1, 2, . . . , N) with a local signal whose frequency is equal to that of the radiated radar wave to produce a beat signal. The beat signals thus produced are inputted to a signal processor 110 through an A/D converter circuit 108. The signal processor 110 performs a phase/weighting operation on the beat signals in a digital form for producing beams. Specifically, the signal processor 110 realizes functions of analog phase shifters installed in each radiating element in a conventional phased array system and of combining outputs of the analog phase shifters in an analog form.
Usually, the DBF requires expression of the beat signals B1 to BN produced by signals received by the receiving antennas AR1 to ARN in baseband complex signal each made of a real signal I and an imaginary signal Q. The receiver 106 and the A/D converter circuit 108, thus, require not only dual channels each made of a mixer and an A/D converter for each receiving antenna, but also increasing of power of the local signals L supplied to the mixers, which will result in an increase in circuit size.
The reason why the DBF requires the baseband complex signals is because it is difficult to specify the phase of a baseband scalar signal derived only by a received signal in one channel at any time, thus resulting in a difficulty in determining the direction of incoming of a return of the radar wave based on results of operations performed in the DBF.
FIG. 6(a) illustrates signals received by antennas each expressed in a baseband complex signal made up of a real signal I and an imaginary signal Q and corresponding baseband scalar signals each expressed in vector in a first case where a radar return enters a plane of an array of antennas at an angle of axc2x0 to a line perpendicular to the plane from a right direction, as viewed in the drawing. FIG. 6(b) illustrates for a second case where a radar return enters the antenna-arrayed plane at an angle of xe2x88x92axc2x0 to the line perpendicular to the plane from a left direction, as viewed in the drawing. As clearly shown in the drawings, each baseband complex signal in the first case has a sign reverse to that of a corresponding one of the baseband complex signals in the second case, thereby enabling the radar returns to be discriminated between the first and second cases. It is, however, impossible to use the baseband scalar signals each made up of only the real signal I to discriminate the radar returns between the first and second cases.
In other words, the vector of each baseband scalar signal becomes, as shown in FIG. 7, equivalent to that of a corresponding one of the baseband complex signal in a case where radar returns having the same level enter the antenna-arrayed plane from two directions of xc2x1axc2x0. Therefore, if the complex Fourier transform is performed on the baseband scalar signals in the direction of a spatial axis to form the beams, it may cause peaks to appear on resultant frequency components in the both directions of xc2x1axc2x0, which leads to a difficulty in determining whether the baseband scalar signals are produced by the radar return traveling from the direction of +axc2x0 or xe2x88x92axc2x0 or to an erroneous decision that two signals traveling from the both directions of xc2x1axc2x0 have entered the antennas simultaneously.
There has been also proposed a technique for digitizing received signals using high frequency A/D converters before the beat signals undergo a frequency conversion and realizing a two-channel mixer function in a computer through digital signal processing. For example, Japanese Patent First Publication No. 10-63645 teaches such a technique. This, however, requires a large number of expensive A/D converters, thus resulting in an increase in total production cost of the system.
Further, precise measurement of the azimuth or angular direction of a target object within a limited angular range generally requires use of a large number of antennas (i.e., received signals). This also requires many receivers, thus resulting in an increased size of the system.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a radar system having a simple structure which is smaller in load on operations to form digital beams and analyze the frequency of each digital beam.
It is a further object of the invention to provide a radar system which has a compact structure capable of measuring the azimuth of a target with high accuracy.
According to one aspect of the invention, there is provided a radar apparatus which comprises: (a) a transmitter providing a transmit signal having a preselected frequency to produce an output signal to be transmitted as a radar wave to a radar detectable zone; (b) an array of receiving antennas; (c) a plurality of receivers each of which mixes an input signal that is a return of the radar wave from a target object received by one of the receiving antennas with a local signal having the same frequency as that of the transmit signal to produce a single beat signal including a frequency component corresponding to a difference in frequency between the output signal and the input signal; and (d) a signal processing circuit forming beams made of components of the beat signals corresponding to angular directions predetermined in the radar detectable zone, the signal processing performing complex Fourier Transform twice on the beat signals in time series and space series along the array of the receiving antennas to derive results of frequency analysis in units of the beams.
In the preferred mode of the invention, the signal processing circuit adds a plurality of dummy data whose values are zero to the beat signals when the Fourier Transform is performed in the space series so as to produce frequency components greater in number than the receivers.
Each of the receivers receives the input signal from one of the receiving antennas arrayed in line. The signal processing circuit performs the complex Fourier Transform on each of a first beat signal group made up of the beat signals produced by the input signals from the antennas other than one located at one end of the array of the antennas and a second beat signal group made up of the beat signals produced by the input signals from the antennas other than one located at the other end of the array of the antennas to form the beams in units of the first and second beat signal groups.
The signal processing circuit performs the complex Fourier Transform, in the time series, on the beat signal produced by each of the receivers to produce frequency components and also performs the complex Fourier Transform, in the space series, on the frequency components produced using the beat signals in all the receivers in units of frequency for forming the beams.
The transmitter produces the transmit signal whose frequency is increased and decreased cyclically.
The transmitter produces, in sequence, a plurality of transmit signals having different frequencies.